Structural requirements for species-specific induction of the sperm acrosome reaction by sea urchin egg sulfated fucan

The sulfated fucan (SF) of egg jelly induces the acrosome reaction (AR) of sea urchin sperm. Strongylocentrotus franciscanus (Sf) SF is sulfated only at the 2-position. Strongylocentrotus purpuratus (Sp) has two SF isotypes, each one being female specific. One is rich in sulfate at both the 2- and 4-positionS (SF-1), and the other is rich in sulfate at the 4-position, but not the 2-position (SF-2). Sf SF is poor at inducing the AR of Sp sperm, presumably due to lack of 4-sulfation. Sp SF-1 is better at inducing the AR of Sf sperm than Sp SF-2, hypothetically due to increased 2-sulfation. Chemical oversulfation of Sf SF increases the percentage of AR of Sp sperm, showing that 4-sulfation is important for recognition of SF by Sp sperm. Chemically oversulfated Sp SF-2 is better at inducing the Sf sperm AR, presumably because of increased 2-sulfation. The species, Strongylocentrotus drobachiensis (Sd), has an SF-2 that is exclusively 2-sulfated (like Sf), except the glycosidic linkage in Sd is alpha(1-->4), whereas in Sf it is alpha(1-->3). Sd SF-2 does not induce the AR of Sf sperm, showing the strict requirement for the alpha(1-->3) linkage in recognition between Sf sperm and SF. Egg jelly from Echinometra lucunter (El) contains sulfated galactan (SG) which differs from Sf SF only in that the monosaccharide is L-galactose, not L-fucose. This SG and Sf SF are equally potent in inducing the AR of Sf sperm, showing that modification at C6 of L-fucose is not important for proper recognition between SF and Sf sperm receptors. This system permits study of the structural basis for recognition between sulfated polysaccharide and receptors controlling signal transduction pathways in animal cells.

An ATP-binding cassette transporter is a major glycoprotein of sea urchin sperm membranes

Sperm are terminally differentiated cells that undergo several membrane-altering events before fusion with eggs. One event, the sea urchin sperm acrosome reaction (AR), is blocked by the lectin wheat germ agglutinin (WGA). In an effort to identify proteins involved in the AR induction, the peptide sequence was obtained from a 220-kDa WGA-binding protein. Degenerate PCR and library screening resulted in the full-length deduced amino acid sequence of an ATP-binding cassette transporter, suABCA. The protein of 1,764 residues has two transmembrane regions, two nucleotide-binding domains, and is most closely related to the human ABC subfamily A member 3 transporter (ABCA3). Sequence analysis suggests a large extracellular loop between transmembrane spanning segments 7 and 8, with five N-linked glycosylation sites. An antibody made to the loop region binds to non-permeabilized cells, supporting that this region is extracellular. suABCA is found in sperm membrane vesicles, it can be solubilized with nonionic detergents, and it shifts from 220 to 200 kDa upon protein:N-glycanase F digestion. suABCA localizes to the entire surface of sperm in a punctate pattern, but is not detected in lipid rafts. Based on its relationship to subfamily A, suABCA is most likely involved in phospholipid or cholesterol transport. This is the first investigation of an ABC transporter in animal sperm.

Egg fucose sulfate polymer, sialoglycan, and speract all trigger the sea urchin sperm acrosome reaction

Macromolecules surrounding eggs induce the acrosome reaction (AR) of spermatozoa. In sea urchins, three egg jelly (EJ) molecules: a fucose sulfate polymer (FSP), a sialoglycan (SG), and speract mediate ionic fluxes triggering the AR. SG and speract are noninductive without FSP. Speract's role in AR induction is controversial. Here we show that speract potentiates the FSP-induced AR at pH 7.0, approximately 1 pH unit lower than natural seawater. At pH 7.0, a mixture of FSP, SG, and speract produces the intracellular pH increase necessary for maximum AR induction. Each EJ component may mediate a distinct intracellular pH control mechanism, and all three may function synergistically to increase the intracellular pH permitting AR induction. Speract peptides are an ancient family. Although important for activating cyclic nucleotide-mediated pathways in today's seawater of pH approximately 8, speract may have been more important in AR induction in the paleo-ocean of pH approximately 7.

A flagellar K(+)-dependent Na(+)/Ca(2+) exchanger keeps Ca(2+) low in sea urchin spermatozoa

The metabolism, flagellar beating, and acrosome reaction of spermatozoa are regulated by ion flux across the plasma membrane. As is true of most cells, swimming sperm maintain intracellular Ca(2+) concentrations at submicromolar levels. Here we describe a K(+)-dependent Na(+)/Ca(2+) exchanger (suNCKX) from sea urchin sperm. The suNCKX is phylogenetically related to other NCKXs, which use high relative intracellular K(+), and high relative extracellular Na(+), to couple the efflux of 1 Ca(2+) and 1 K(+) to the influx of 4 Na(+). The 652-aa suNCKX shares structural topology with other NCKX proteins, and has two protein kinase A sites and a His-rich region in its cytoplasmic loop. The suNCKX is encoded by a single gene, which is highly expressed in testes. The suNCKX activity of whole sperm shows Na(+) and K(+) dependence, and like other NCKXs can run in reverse exchange mode. An inhibitor blocks the suNCKX activity and sperm motility. suNCKX localizes to the plasma membrane over the sperm flagellum. The suNCKX may play a major role in keeping Ca(2+) low in swimming sperm.

Full-length sequence of VERL, the egg vitelline envelope receptor for abalone sperm lysin

Abalone sperm use 16 kDa lysin to create a hole in the egg vitelline envelope (VE) by a species-specific, nonenzymatic mechanism. To create the hole, lysin binds tightly to VERL (the VE receptor for lysin), a giant, unbranched glycoprotein comprising 30% of the VE. Binding of lysin to VERL causes the VERL molecules to lose cohesion and splay apart creating the hole. Lysin and VERL represent a cognate pair of gamete recognition proteins, one male the other female, which mediate fertilization. The coevolution of such cognate pairs may underlie the establishment of species-specific fertilization which could be a component of the mechanism to achieve reproductive isolation and hence new species. Here we present the full-length cDNA sequence (11,166 bp) of VERL from the red abalone (Haliotis rufescens). There are 42 amino acids from the start Met residue to the beginning of the first 'VERL repeat'. Most of VERL (9981 bp; 89.4%) consists of 22 tandem repeats of a approximately 153 amino acid sequence that is predicted to be beta-sheet. The last VERL repeat is followed by 353 non-repeat amino acid residues containing a furin cleavage site (RTRR), a ZP domain and a hydrophobic COOH-terminus with a 3' UTR of only 10 nucleotides. VERL repeats 3-22 have been subjected to concerted evolution and consequently have almost identical sequences. Curiously, comparisons of repeats from other species shows that repeats 1 and 2 of red abalone VERL have not been subjected to concerted evolution since the divergence of the red species from the other six California species.

Egg sialoglycans increase intracellular pH and potentiate the acrosome reaction of sea urchin sperm

Sea urchin egg jelly (EJ) triggers sperm acrosome reaction (AR), an exocytotic event required for membrane fusion of the gametes. Purified fucose sulfate polymer (FSP) in EJ is one inducer of the AR. Binding of FSP to its receptor regulates opening of two distinct calcium channels and also elevates intracellular pH (pH(i)). EJ also contains sialic acid-rich glycans (sialoglycans (SG)) that were isolated by beta-elimination followed by DEAE chromatography. In the presence of limiting amounts of FSP, the SG fraction markedly potentiates the AR; however, by itself SG has no activity. The SG fraction increases the pH(i) of sperm without increasing intracellular Ca(2+). The SG-induced increase in pH(i) is not blocked by nifedipine or high K(+), whereas the FSP-induced pH(i) increase is sensitive to both these agents. Treatment of the SG fraction with neuraminidase or mild metaperiodate that specifically cleaves the glycerol side chain of sialic acid abolishes the AR potentiation and ability of SG to elevate pH(i). These data are the first to show that there are at least two pathways to induce sperm pH(i) increase and that egg surface sialic acid plays a role in triggering the sperm AR.

The rapid evolution of reproductive proteins

Many genes that mediate sexual reproduction, such as those involved in gamete recognition, diverge rapidly, often as a result of adaptive evolution. This widespread phenomenon might have important consequences, such as the establishment of barriers to fertilization that might lead to speciation. Sequence comparisons and functional studies are beginning to show the extent to which the rapid divergence of reproductive proteins is involved in the speciation process.

Exploring the phylogenetic utility of ITS sequences for animals: a test case for abalone (Haliotis)

To evaluate the general utility of sequences of the nuclear rDNA internal transcribed spacer (ITS) regions for phylogenetic analyses of animal species groups and their broader relationships, sequences were obtained for 19 species of the genus Haliotis plus a keyhole limpet and a more distantly related gastropod, the Chilean abalone. Three subclades of Haliotis species appear consistently, each encompassing little variation. They are (A) the North Pacific species, (B) the European species, and (C) the Australia species. The one Caribbean species examined clearly groups with the North Pacific clade, not the European clade. H. midae (South Africa) and H. diversicolor supertexta (Taiwan) both diverge basal to the European and Australian species groups in the phylogenetic trees. Sequence comparisons showed that one species of Haliotis, H. iris from New Zealand, is quite distant from the remaining Haliotis species, almost as much as the more obvious outgroup, the keyhole limpet, an observation common to other DNA sequence analyses of these taxa. Using the rate of nucleotide change calculated from the sister Caribbean-Pacific pair, the length of the H. iris long branch is compatible with the suggestion that its ancestry became isolated on New Zealand at Gondwandan breakup. Use of ITS permits a totally independent estimate of the phylogenetic relationships, yet branching order was very similar to that established using other DNA regions studied previously, including those under strong positive selection. Knowledge of the RNA transcript secondary structure is particularly useful in the optimal alignment of more distantly related taxa. The RNA transcript secondary structure of Haliotis ITS2 shows conservation of features found also in ITS2 of angiosperms and algal taxa. Since ITS, particularly ITS2, is not saturated with nucleotide changes even at the family level, it should be useful for phylogenetic reconstruction of animal groups, not just at the species and genus levels but perhaps also for families and above.

suREJ3, a polycystin-1 protein, is cleaved at the GPS domain and localizes to the acrosomal region of sea urchin sperm

The sea urchin sperm acrosome reaction (AR) is a prerequisite for sperm-egg fusion. This report identifies sea urchin sperm receptor for egg jelly-3 (suREJ3) as a new member of the polycystin-1 family (the protein mutated in autosomal dominant polycystic kidney disease). suREJ3 is a multidomain, 2,681-amino acid, heavily glycosylated orphan receptor with 11 putative transmembrane segments (TMS) that localize to the plasma membrane covering the sperm acrosomal vesicle. Like the latrophilins and other members of the secretin family of G-protein-coupled receptors, suREJ3 is cleaved at the consensus GPS (G-protein-coupled receptor proteolytic site) domain. Antibodies to the extracellular 1,455-residue NH(2)-terminal portion identify a band at 250 kDa that shifts in electrophoretic mobility to 180 kDa upon glycosidase digestion. Antibodies to the 1,226-residue COOH-terminal portion identify a band at 150 kDa that shifts to 140 kDa after glycosidase treatment. Antibodies to both portions of suREJ3 localize exclusively to the plasma membrane over the acrosomal vesicle. Immunoprecipitation shows that both portions of suREJ3 are associated in detergent extracts. This is the first report showing that a polycystin family member is cleaved at the GPS domain. Localization of suREJ3 to the acrosomal region provides the first suggestion for the role of a polycystin-1 protein (components of nonselective cation channels) in a specific cellular process.

High molecular mass egg fucose sulfate polymer is required for opening both Ca2+ channels involved in triggering the sea urchin sperm acrosome reaction

A linear fucose sulfate polymer (FSP), >10(6) daltons, is a major component of sea urchin egg jelly. FSP induces the sperm acrosome reaction (AR), an exocytotic process required for animal fertilization. Two Ca(2+) channels activate during AR induction, the first opens 1 s after FSP addition, and the second opens 5 s after the first. Mild acid hydrolysis of FSP results in a linear decrease in polymer size. The ability of FSP to induce the AR and activate sperm Ca(2+) channels decreases with increasing time of hydrolysis. Hydrolyzed FSP of approximately 60 kDa blocks intact FSP from inducing the AR. At 44 microg/ml hydrolyzed FSP, Ca(2+) entry into sperm is almost equal to that occurring in 3.8 microg/ml intact FSP; however the AR is not induced. The shape of the [Ca(2+)](i) increase curve and use of the Ca(2+) channel blockers nifidipine and Ni(2+) indicate that hydrolyzed FSP opens the second Ca(2+) channel, but not the first, and thus does not induce the AR. The giant size of intact FSP is required to open both Ca(2+) channels involved in triggering the AR.

Embryos of the sea urchin Strongylocentrotus purpuratus synthesize a dermatan sulfate enriched in 4-O- and 6-O-disulfated galactosamine units

Unfertilized eggs of the sea urchin Strongylocentrotus purpuratus are surrounded by a gelatinous layer rich in sulfated fucan. Shortly after fertilization this polysaccharide disappears, but 24 h later the embryos synthesize high amounts of dermatan sulfate concomitantly with the mesenchyme blastula-early gastrula stage when the larval gut is forming. This glycosaminoglycan has the same backbone structure [4-alpha-L-IdoA-1-->3-beta-D-GalNAc-1](n) as the mammalian counterpart but possesses a different sulfation pattern. It has a high content of 4-O- and 6-O-disulfated galactosamine units. In addition, chains of this dermatan sulfate are considerable longer than those of vertebrate tissues. Adult sea urchin tissues contain high concentrations of sulfated polysaccharides, but dermatan sulfate is restricted to the adult body wall where it accounts for approximately 20% of the total sulfated polysaccharides. In addition, sulfation at the 4-O-position decreases markedly in the dermatan sulfate from adult sea urchin when compared with the glycan from larvae. Overall, these results demonstrate the occurrence of dermatan sulfates with unique sulfation patterns in this marine invertebrate. The physiological implication of these oversulfated dermatan sulfates is unclear. One hypothesis is that interactions between components of the extracellular matrix in marine invertebrates occur at higher salt concentrations than in vertebrates and therefore require glycosaminoglycans with increased charge density.

The crystal structure of a fusagenic sperm protein reveals extreme surface properties

Sp18 is an 18 kDa protein that is released from abalone sperm during the acrosome reaction. It coats the acrosomal process where it is thought to mediate fusion between sperm and egg cell membranes. Sp18 is evolutionarily related to lysin, a 16 kDa abalone sperm protein that dissolves the vitelline envelope surrounding the egg. The two proteins were generated by gene duplication followed by rapid divergence by positive selection. Here, we present the crystal structure of green abalone sp18 resolved to 1.86 A. Sp18 is composed of a bundle of five alpha-helices with surface clusters of basic and hydrophobic residues, giving it a large dipole moment and making it extremely amphipathic. The large clusters of hydrophobic surface residues and domains of high positive electrostatic surface charge explain sp18's ability as a potent fusagen of liposomes. The overall fold of sp18 is similar to that of green abalone lysin; however, the surface features of the proteins are quite different, accounting for their different roles in fertilization. This is the first crystal structure of a protein implicated in sperm-egg fusion during animal fertilization.

Glycobiology of sperm-egg interactions in deuterostomes

The process of fertilization begins when sperm contact the outermost egg investment and ends with fusion of the two haploid pronuclei in the egg cytoplasm. Many steps in fertilization involve carbohydrate-based molecular recognition between sperm and egg. Although there is conservation of gamete recognition molecules within vertebrates, their homologues have not yet been discovered in echinoderms and ascidians (the invertebrate deuterostomes). In echinoderms, long sulfated polysaccharides act as ligands for sperm receptors. Ascidians employ egg coat glycosides that are recognized by sperm surface glycosidases. Vertebrate egg coats contain zona pellucida (ZP) family glycoproteins, whose carbohydrates bind to sperm receptors. Several candidate sperm receptors for vertebrate ZP proteins have been identified and are discussed here. This brief review focuses on new information concerning fertilization in deuterostomes (the phylogenetic group including echinoderms, ascidians, and vertebrates) and highlights protein-carbohydrate interactions involved in this process.

Polymorphism in abalone fertilization proteins is consistent with the neutral evolution of the egg's receptor for lysin (VERL) and positive darwinian selection of sperm lysin

The evolution of species-specific fertilization in free-spawning marine invertebrates is important for reproductive isolation and may contribute to speciation. The biochemistry and evolution of proteins mediating species-specific fertilization have been extensively studied in the abalone (genus Haliotis). The nonenzymatic sperm protein lysin creates a hole in the egg vitelline envelope by species-specifically binding to its egg receptor, VERL. The divergence of lysin is promoted by positive Darwinian selection. In contrast, the evolution of VERL does not depart from neutrality. Here, we cloned a novel nonrepetitive region of VERL and performed an intraspecific polymorphism survey for red (Haliotis rufescens) and pink (Haliotis corrugata) abalones to explore the evolutionary forces affecting VERL. Six statistical tests showed that the evolution of VERL did not depart from neutrality. Interestingly, there was a subdivision in the VERL sequences in the pink abalone and a lack of heterozygous individuals between groups, suggesting that the evolution of assortative mating may be in progress. These results are consistent with a model which posits that egg VERL is neutrally evolving, perhaps due to its repetitive structure, while sperm lysin is subjected to positive Darwinian selection to maintain efficient interaction of the two proteins during sperm competition.

Abalone lysin: the dissolving and evolving sperm protein

Abalone sperm lysin is a non-enzymatic protein that creates a hole for sperm passage in the envelope surrounding the egg. Lysin exhibits species-specificity in making the hole and it evolves rapidly by positive selection. Our studies have focused on combining structural, biochemical, and evolutionary data to understand the mechanism of action and evolution of this remarkable protein. Currently, more is known about lysin than about any other protein involved in animal fertilization. We present an hypothesis to explain lysin's rapid evolution and the evolution of species-specific fertilization in this order of mollusks. We also propose a two-step model for lysin's action in which a dimer of lysin binds species-specifically to its glycoprotein receptor, and then monomerizes and binds the receptor in a non-species-specific manner. This experimental system yields data relevant to the general problem of molecular recognition between cell surfaces, and is also important to our thinking about how new species arise in the sea. BioEssays 23:95-103, 2001.

Maximum-likelihood analysis of molecular adaptation in abalone sperm lysin reveals variable selective pressures among lineages and sites

Maximum-likelihood models of codon substitution were used to analyze sperm lysin genes of 25 abalone (HALIOTIS:) species to identify lineages and amino acid sites under diversifying selection. The models used the nonsynonymous/synonymous rate ratio (omega = d(N)/d(S)) as an indicator of selective pressure and allowed the ratio to vary among lineages or sites. Likelihood ratio tests suggested significant variation in selective pressure among lineages. The variable selective pressure provided an explanation for the previous observation that the omega ratio is >1 in comparisons of closely related species and <1 in comparisons of distantly related species. Computer simulations demonstrated that saturation of nonsynonymous substitutions and constraint on lysin structure were unlikely to account for the observed pattern. Lineages linking closely related sympatric species appeared to be under diversifying selection, while lineages separating distantly related species from different geographic locations were associated with low evolutionary rates. The selective pressure indicated by the omega ratio was found to vary greatly among amino acid sites in lysin. Sites under potential diversifying selection were identified. Ancestral lysins were inferred to trace the route of evolution at individual sites and to provide lysin sequences for future laboratory studies.

The high resolution crystal structure of green abalone sperm lysin: implications for species-specific binding of the egg receptor

Abalone sperm lysin is a 16 kDa acrosomal protein used by sperm to create a hole in the egg vitelline envelope. Lysins from seven California abalone exhibit species-specificity in binding to their egg receptor, and range in sequence identity from 63 % to 90 %. The crystal structure of the sperm lysin dimer from Haliotis fulgens (green abalone) has been determined to 1.71 A by multiple isomorphous replacement. Comparisons with the structure of the lysin dimer from Haliotis rufescens (red abalone) reveal a similar overall fold and conservation of features contributing to lysin's amphipathic character. The two structures do, however, exhibit differences in surface residues and electrostatics. A large clustering of non-conserved surface residues around the waist and clefts of the dimer, and differences in charged residues around these regions, indicate areas of the molecule which may be involved in species-specific egg recognition.

Co-localization of receptor and transducer proteins in the glycosphingolipid-enriched, low density, detergent-insoluble membrane fraction of sea urchin sperm

The low density, detergent-insoluble membrane fraction (LD-DIM), where gangliosides are likely to be highly enriched, was prepared from sperm of two sea urchin species, Hemicentrotus pulcherrimus and Strongylocentrotus purpuratus. Immunoblotting showed the presence in the LD-DIM of two receptors for egg ligands, a glycosylphosphatidylinositol (GPI)-anchored protein, and four proteins which may be involved in signal transduction. Co-immunoprecipitation revealed that at least three proteins, the speract receptor, the 63kDa GPI-anchored protein and the alpha subunit of a heterotrimeric Gs protein, are localized in the LD-DIM. This suggests that the LD-DIM fraction may be a membrane microdomain for speract-speract receptor interaction, as well as the subsequent signal transduction pathway involved in induction of sperm respiration, motility and possibly the acrosome reaction.

Positive selection and propeptide repeats promote rapid interspecific divergence of a gastropod sperm protein

Male-specific proteins have increasingly been reported as targets of positive selection and are of special interest because of the role they may play in the evolution of reproductive isolation. We report the rapid interspecific divergence of cDNA encoding a major acrosomal protein of unknown function (TMAP) of sperm from five species of teguline gastropods. A mitochondrial DNA clock (calibrated by congeneric species divided by the Isthmus of Panama) estimates that these five species diverged 2-10 MYA. Inferred amino acid sequences reveal a propeptide that has diverged rapidly between species. The mature protein has diverged faster still due to high nonsynonymous substitution rates (> 25 nonsynonymous substitutions per site per 10(9) years). cDNA encoding the mature protein (89-100 residues) shows evidence of positive selection (Dn/Ds > 1) for 4 of 10 pairwise species comparisons. cDNA and predicted secondary-structure comparisons suggest that TMAP is neither orthologous nor paralogous to abalone lysin, and thus marks a second, phylogenetically independent, protein subject to strong positive selection in free-spawning marine gastropods. In addition, an internal repeat in one species (Tegula aureotincta) produces a duplicated cleavage site which results in two alternatively processed mature proteins differing by nine amino acid residues. Such alternative processing may provide a mechanism for introducing novel amino acid sequence variation at the amino-termini of proteins. Highly divergent TMAP N-termini from two other tegulines (Tegula regina and Norrisia norrisii) may have originated by such a mechanism.

1.35 and 2.07 A resolution structures of the red abalone sperm lysin monomer and dimer reveal features involved in receptor binding

Abalone sperm use lysin to make a hole in the egg's protective vitelline envelope (VE). When released from sperm, lysin first binds to the VE receptor for lysin (VERL) then dissolves the VE by a non-enzymatic mechanism. The structures of the monomeric and dimeric forms of Haliotis rufescens (red abalone) lysin, previously solved at 1.90 and 2.75 A, respectively, have now been refined to 1.35 and 2.07 A, respectively. The monomeric form of lysin was refined using previously obtained crystallization conditions, while the dimer was solved in a new crystal form with four molecules (two dimers) per asymmetric unit. These high-resolution structures reveal alternate residue conformations, enabling a thorough analysis of the conserved residues contributing to the amphipathic nature of lysin. The availability of five independent high-resolution copies of lysin permits comparisons leading to insights on the local flexibility of lysin and alternative conformations of the hypervariable N-terminus, thought to be involved in species-specific receptor recognition. The new analysis led to the discovery of the basic nature of a cleft formed upon dimerization and a patch of basic residues in the dimer interface. Identification of these features was not possible at lower resolution. In light of this new information, a model explaining the binding of sperm lysin to egg VERL and the subsequent dissolution of the egg VE is proposed.

Interspecies chimeric sperm lysins identify regions mediating species-specific recognition of the abalone egg vitelline envelope

Abalone sperm lysin is a nonenzymatic, 16-kDa protein that creates a hole in the egg vitelline envelope (VE) through which the sperm swims to fuse with the egg. The dissolution of isolated VE by lysin is species specific. Interspecies comparisons show that the most divergent region of lysin is the N-terminal segment of residues 1-12 which is always species-unique. The C-terminus and three internal segments are moderately variable between species, but not species unique. Analysis of nucleotide substitutions shows that lysin evolves rapidly by positive Darwinian selection, suggesting that there is adaptive value in altering its amino acid sequence. The results reported here, in which segments of lysin were exchanged between two species, prove by direct experimentation that the interspecies variable termini play major roles in the species-specific recognition between sperm lysin and the egg VE.

Structure of the sulfated alpha-L-fucan from the egg jelly coat of the sea urchin Strongylocentrotus franciscanus: patterns of preferential 2-O- and 4-O-sulfation determine sperm cell recognition

The egg jelly coats of sea urchins contains sulfated polysaccharides responsible for inducing the sperm acrosome reaction which is an obligatory event for sperm binding to, and fusion with, the egg. Here, we extend our study to the sea urchin Strongylocentrotus franciscanus. The egg jelly of this species contains a homofucan composed of 2- O -sulfated, 3-linked units which is the simplest structure ever reported for a sulfated fucan. This polysaccharide was compared with other sulfated alpha-L-fucans as inducers of acrosome reaction in conspecific and heterospecific sperm. Although all these fucans are linear polymers composed of 3-linked alpha-L-fucopyranosyl units, they differ in the proportions of 2-O- and 4-O-sulfation. The reactivity of the sperm of each species is more sensitive to the egg jelly sulfated fucan found in their own species. The reactivity of the sperm does not correlate with the charge density of the fucan, but with the proportion of 2-O- and 4-O-sulfation. The pattern of sulfation may be an important feature for recognition of fucans by the sperm receptor contributing to the species-specificity of fertilization.